ipn/ipnlocal: do unexpired cert renewals in the background

We were eagerly doing a synchronous renewal of the cert while
trying to serve traffic. Instead of that, just do the cert
renewal in the background and continue serving traffic as long
as the cert is still valid.

This regressed in c1ecae13ab when
we introduced ARI support and were trying to make the experience
of `tailscale cert` better. However, that ended up regressing
the experience for tsnet as it would not always doing the renewal
synchronously.

Fixes #9783

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/localapi/cert.go

@@ -23,7 +23,7 @@ func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "internal handler config wired wrong", 500)
 		return
 	}
-	pair, err := h.b.GetCertPEM(r.Context(), domain, true)
+	pair, err := h.b.GetCertPEM(r.Context(), domain)
 	if err != nil {
 		// TODO(bradfitz): 500 is a little lazy here. The errors returned from
 		// GetCertPEM (and everywhere) should carry info info to get whether